How to compare three (or n) values and output the minimum value using nodes
Shader nodes?  SeparateXYZ (or SeparateRGB, it doesn't matter, they do the same thing) and then run serial minimum operations:

Dividing by 255, because your numbers suggest you're interpreting color as values in 0,255 when Blender interprets colors as 0,1.
Serial maximum operations will work the same way.
If you want to, you can node group all of that stuff to make a MinimumVec node group, for easy reuse.